14. Navigation Guidance and Control System

Abstract

The navigation, guidance and control (NGC) system, the brain of the vehicle, is responsible for directing the propulsive forces and stabilizing the vehicle along the desired path to achieve the orbit with the specified accuracy. The NGC system has to define the optimum trajectory in real time to reach the specified target and steer the vehicle along the desired path and inject the spacecraft into the mission targeted orbit within the specified dispersions. The navigation system measures the instantaneous state of the vehicle, and using this information, the guidance system generates the optimum trajectory to achieve the target and desired vehicle steering command to realize the optimum trajectory in real time. The vehicle control system, comprising of autopilot and control power plants, receives the steering commands from the guidance system and steers the vehicle to follow the desired attitude in the presence of all disturbances. The guidance system charts out the remaining path continuously, recalculating the desired attitude of the vehicle to achieve the mission target. Final mission objectives and allowable orbital dispersions dictate the choice of a suitable guidance system. The vehicle autopilot has three major functions namely, to ensure that the vehicle loads are always well within the specified limits, to stabilize the vehicle all through the flight and to steer the vehicle to follow the desired attitude as decided by the guidance system. The autopilot generates the control commands as per the defined control law which is used to drive the actuation systems to generate the necessary control forces. Proper choice of inertial sensors, inertial systems, guidance schemes, control actuation systems and control laws depend on the mission objectives and requirements. This chapter starts with the functional requirements and systems requirements of NGC system of a launch vehicle. The need for the integrated design of NGC system using systems engineering approach is explained next. Various elements involved in NGC system like navigation, guidance and control, the selection of a suitable scheme and their design aspects are also explained. The performance evaluation of the integrated design of NGC system carried out using different test beds is described.